How exactly are discs rated at different speeds physically different?

Example: How is a Speer DVD-R ×4 physically different than a GSC003 (×8) or a PLATINUM (RiTEKF01) DVD-R×16?

Which physical difference limits the former ones compared to the ×16?

The main reason is the recording dye formulation. If I remember correctly low speed disc have dye created for low power/long exposure to the laser beam where high speed disc have dye formulation for high power/short exposure.

RITEKF01 = Ritek media with Fuji dye (the F part) formulated for 16x recording (the 01 part).

I noticed in FW media support list that some drives support recording to RITEKF02, F03 discs but I’ve never seen one in person. I’m guessing those are disc optimized for 8X or 4X recording based around the same Fuji dye.

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Don’t WORM discs (-R/+R) only have a maximum speed?

RW discs have a limited speed range. CD-RWhs: ×8 to ×12, burners chose ×10.

Unofficially 16x media has minimum speed limit as well, usually it is 4x. Going bellow 4x (if supported by the burner) is usually worse than going over 16x.

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CD-R has a giant speed range however; starting with ×1 or ×2, all the way up to ×48/×52/×56.

The same applies for CD-R media - officially range for modern media usually is from 1x to 52x but the optimum range is somewhere between 16x -32x. If you try burning at 1x or 2x probably the burn will fail.

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Good to know, I will do some tests on CD-R SQ (cheap eNay Shit(ty)Quality).